This invention relates to microwave waveguide filters and, more particularly, to a compact design that is readily manufactured.
Satellite communications systems relay the communications signals as microwaves. A microwave communications signal is up-transmitted from a first earth station to the communications satellite, processed on board the satellite, and down-transmitted to a second earth station. Typically, many channels of communications signals are relayed simultaneously.
The on-board processing of the communications signals usually involves filtering the microwave communications signals, amplifying the signals, and possibly other signal conditioning. Because many channels are transmitted simultaneously and because the communications are subject to various types of interference, it is important that the microwave signals be filtered to remove noise and any undesirable components, and to ensure separation between the signal bands.
On-board microwave signals may be propagated in any suitable fashion. The main approaches are within waveguides, on striplines, and between coaxial conductors. Each of these propagation media has filters available. The present invention is concerned with one of these, the microwave waveguide filter.
The usual approach to the microwave waveguide filter is to provide suitably configured and sized cavities in the waveguide. Resonant modes are produced in the cavities, with the result that the microwave energy leaving the microwave waveguide filter is filtered responsive to the configuration and size of the cavity or cavities. Such microwave waveguide filters are operable and are widely used, but they have drawbacks. The existing designs are usually relatively complex structures that are difficult and expensive to manufacture, with high piece counts, resulting in expensive and time-consuming assembly. They may also be difficult, time consuming, and expensive to tune property and to maintain tuned.
There is therefore a need for an improved design for a microwave waveguide filter. The present invention fulfills this need, and further provides related advantages.
The present invention provides a microwave waveguide filter for quasi-elliptical filtering of microwave signals. The microwave waveguide filter is readily and inexpensively manufactured, and has a low piece count of parts. Additionally, the filtering performance of the design is readily predicted theoretically, reducing the trial-and-error, and thus the time and expense, to tune the filter performance. The design is particularly suited for cross coupled cavity resonator filters for use in the K band and at higher frequencies.
In accordance with the invention, a microwave waveguide filter comprises a main-line cavity structure comprising a group of at least two rectangular main-line cavities arrayed along a main propagation path and including a first main-line cavity and a second main-line cavity. Each main-line cavity includes a sidewall. Each pair of adjacent main-line cavities has a common transverse wall therebetween transverse to (and preferably perpendicular to) the main propagation path, and a main-line aperture in the common transverse wall. There is a rectangular first feedback cavity in microwave communication with each of the first main-line cavity and the second main-line cavity through the respective sidewall of the first main-line cavity and the second main-line cavity. Thus, there is a first-cavity feedback aperture between the first feedback cavity and the first main-line cavity, and a second-cavity feedback aperture between the first feedback cavity and the second main-line cavity.
Preferably, the main-line cavities and the first feedback cavity have a base wall (i.e., a floor) that lies in a common filter plane. The main-line cavity structure may be linear and unfolded, so that the main propagation path is substantially a straight line. The main-line cavity structure may instead be nonlinear and folded, so that the main propagation path is not substantially a straight line.
In one embodiment, the main-line cavity structure includes an input-end main-line cavity at a first end of the main-line cavity structure, and an output-end main-line cavity at a second end of the main-line cavity structure. The main-line cavity structure further includes an input structure in microwave communication with the input-end main-line cavity, and an output structure in microwave communication with the output-end main-line cavity.
The size of the feedback cavity is selected to provide the desired filtering. In an example, a first-cavity sidewall of the first main-line cavity and a second-cavity sidewall of the second main-line cavity are parallel (and preferably coplanar) and both of a first-sidewall length. The first feedback cavity has a first-feedback-cavity sidewall that is parallel to the first-cavity sidewall and the second-cavity sidewall. The first-feedback-cavity sidewall has a first-feedback-cavity-sidewall length of about the first-sidewall length in one embodiment, and the first-feedback-cavity-sidewall length of about two times the first-sidewall length in another embodiment.
Most conveniently, the main-line cavity structure and the first feedback cavity are formed in a single filter block of material, as by machining and preferably by milling. A single cover is provided to overlie the machined-out main-line cavity structure and to be affixed to the single filter block of material. With this approach, a second microwave waveguide filter may be readily machined into the opposing side of the single filter block of material, in a back-to-back relation to the microwave waveguide filter.
The main-line cavity structure may be extended to include a third main-line cavity, a fourth main-line cavity, and additional main-line cavities as desired. One reason to extend the main-line cavity structure is to add one or more additional feedback cavities. For example, the main-line cavity structure may include a rectangular second feedback cavity in microwave communication with each of the third main-line cavity and the fourth main-line cavity through the respective sidewall of the third main-line cavity and the fourth main-line cavity. In this case there would be a third-cavity feedback aperture between the second feedback cavity and the third main-line cavity, and a second-cavity feedback aperture between the second feedback cavity and the fourth main-line cavity. As with the embodiment having a single feedback cavity, it is preferred that each of the main-line cavities, the first feedback cavity, and the second feedback cavity share a base wall that lies in a common filter plane. The base wall is preferably the bottom of the single filter block of material. The second-feedback-cavity-sidewall length is selected in the same manner as described above. The two feedback cavities may be dimensioned similarly for redundant filtering, or differently for filtering different microwave modes.
A preferred method for fabricating a microwave waveguide filter comprises the steps of providing a single filter block of material, and fabricating the single filter block of material to have therein a main-line cavity structure comprising a group of at least two rectangular main-line cavities arrayed along a main propagation path and including a first main-line cavity and a second main-line cavity. Each main-line cavity includes a sidewall, and each pair of adjacent main-line cavities has a common transverse wall therebetween transverse to, and preferably perpendicular to, the main propagation path, and a main-line aperture in the common transverse wall. There is a rectangular first feedback cavity in microwave communication with each of the first main-line cavity and the second main-line cavity through the respective sidewall of the first main-line cavity and the second main-line cavity. A first-cavity feedback aperture opens between the first feedback cavity and the first main-line cavity, and a second-cavity feedback aperture opens between the first feedback cavity and the second main-line cavity. This main-line cavity structure is preferably machined, as by numerically controlled milling, into the single filter block of material. Consistent features discussed above may be used in conjunction with the method.
The present approach provides sign change coupling between adjacent cavities without any conductive probe extending between the adjacent cavities. In an alternative approach to a microwave waveguide filter that is not within the scope of the invention, a conductive probe extends between adjacent cavities (and without any aperture between the adjacent cavities). The conductive probe usually includes an electrically conductive rod or wire extending between the adjacent cavities, supported in an annular insulator that fills a hole in the wall between the cavities. This conductive probe achieves capacitive coupling between the adjacent cavities, but it requires two parts that must be produced and assembled for each such conductive probe. Additionally, the length of the conductive probe in each of the adjacent cavities must be fine tuned. In the present approach, on the other hand, there is no conductive probe extending between the cavities, and instead the microwave signal is communicated between adjacent cavities by an aperture that provides inductive coupling. Thus, the presently preferred approach vastly simplifies the fabrication time and cost of the microwave waveguide filter both by avoiding the use of conductive probes, and by the ability to fabricate the cavity structure, including both the walls and the apertures, in the single filter block of material. The filter block may be stacked with other filter blocks to form a stacked multichannel filter structure that is efficient from both a weight and a volumetric standpoint. Filter performance, such as for the TE101 and T102 modes discussed subsequently, is excellent.
The microwave performance of the array of rectangular cavities is readily modeled, so that its performance, and the precise configuration and dimensions required to produce a desired performance, may be predicted. The absolute dimensional lengths of the various walls are determined responsive to the microwave frequencies to be transmitted through the microwave waveguide filter. The present design approach then permits the microwave waveguide filter to be manufactured inexpensively and precisely to the required configurations, dimensions, and tolerances. The amount of fine tuning that is required to achieve the desired performance is therefore minimal, and may be accomplished, for example, by setting one or more tuning screws that extend through the cover of the main-line cavity structure.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.